The hydrofoil vehicle is analagous to an aircraft, where the wings operate under water. The basic principle of the hydrofoil concept is to lift a craft's hull out of the water and support it dynamically on the submerged wings, i.e. hydrofoils. The hydrofoils can reduce the effect of waves on the craft and reduce the power required to attain modestly high speeds. As the craft's speed is increased the water flow over the hydrofoils increase, generating a lifting force and causing the craft to rise. For a given speed the craft will rise until the lifting force produced by the hydrofoils equals the weight of the craft.
In a typical arrangement, struts connect the hydrofoils to the craft's hull, where the struts have sufficient length to support the hull free of the water surface when operating at cruise speeds. As shown in FIGS. 1a–1c, the basic choices in hydrofoil and strut arrangement are conventional, canard, or tandem. In an example of a conventional arrangement, as shown in FIG. 1b, a pair of struts and hydrofoils are positioned fore of the craft's center of gravity, symmetrical about the craft's longitudinal centerline, and a single strut and hydrofoil is positioned aft of the craft's center of gravity along the craft's longitudinal centerline. In a canard arrangement, as shown in FIG. 1c, a single strut and hydrofoil is positioned fore of the craft's center of gravity along the craft's longitudinal centerline, and a pair of struts and hydrofoils are positioned aft of the craft's center of gravity, symmetrical about the craft's longitudinal centerline.
Alternatively, the pairs of struts can include a single hydrofoil, spanning the beam of the craft. Generally, craft are considered conventional or canard if 65% or more of the weight is supported on the fore or the aft foil respectively.
In a tandem arrangement, as shown in FIG. 1a, pairs of struts and hydrofoils are positioned fore and aft of the craft's center of gravity and symmetrically about the craft's longitudinal centerline. Alternatively, the pairs of struts can include a single hydrofoil, spanning the beam of the craft. If the weight is distributed relatively evenly on the fore and aft hydrofoils, the configuration would be described as tandem.
The hydrofoil's configuration on the strut can be divided into two general classifications, fully submerged and surface piercing. Fully submerged hydrofoils are configured to operate at all times under the water surface. The principal and unique operational capability of craft with fully submerged hydrofoils is the ability to uncouple the craft to a substantial degree from the effect of waves. This permits a hydrofoil craft to operate foil borne at high speed in sea conditions normally encountered while maintaining a comfortable motion environment.
However, the fully submerged hydrofoil system is not self-stabilizing. Consequently, to maintain a specific height above the water, and a straight and level course in pitch and yaw axes, usually requires an independent control system. The independent control system varies the effective angle of attack of the hydrofoils or adjusts trim tabs or flaps mounted on the foils, changing the lifting force in response to changing conditions of craft speed, weight, and sea conditions.
In the surface piercing concept, portions of the hydrofoils are configured to extend through the air/sea interface when foil borne. As speed is increased, the lifting force generated by the water flow over the submerged portion of the hydrofoils increases, causing the craft to rise and the submerged area of the foils to decrease. For a given speed the craft will rise until the lifting force produced by the submerged portion of the hydrofoils equals the weight of the craft. However, because a portion of the surface-piercing hydrofoil is always in contact with the water surface, and therefore the waves, the surface-piercing foil is susceptible to the adverse affect of wave action. The impact of the waves can impart sudden, large forces onto the struts and craft, resulting in an erratic and dangerous motion environment.
Additionally, hydrofoil configurations can include a stack foil, or ladder foil, arrangement, where upper foils are used to provide lift at lower speed, initially raising the craft above the waterline. As the craft's speed is increased, the lower foils produce sufficient lift to support the weight of the craft, further raising the upper foils above the waterline to the cruise height. However, when a wave impacts the craft the upper foil can be instantaneously wetted, producing a sudden increase in lift. The sudden increase in lift produces a jarring impact on the craft, and in some instance can be sufficient enough to instantaneously raise the entire craft, including the main foils, above the waterline.
A hydrofoil vehicle is configured to operate at a particular cruise speed. The cruise speed is the speed at which the total lifting force produced by the hydrofoils equals the all up weight of the hydrofoil vehicle. Operating at speeds greater than the cruise speed can cause the hydrofoils to produce excessive lift, resulting in a cyclic skipping action. At speeds less than the cruise speed, when the hydrofoils do not produce sufficient lift to raise vehicle results in the hull crashing into the water.
Propulsion systems for hydrofoil vehicles can include both water and air propulsion systems. In an exemplary arrangement of a water propulsion system, a water propeller provides the propulsive force, where a drive shaft operably connects the water propeller to an engine. Alternatively, a water jet can be used to provide the propulsive force, where water is funneled through a water intake into the water jet. The water jet accelerates the water, expelling the water through the outlet creating a propulsive force. Air propulsion systems can include for example, air propeller or jet engines. As shown in U.S. Pat. No. 4,962,718 to Gornstein et al., an air propeller is positioned on the deck of the craft and operatively connected to an engine.